Process for the polymerization of olefins

ABSTRACT

Process for the polymerization of olefins CH =CHR, in which R is hydrogen or a hydrocarbon radical with 1-12 carbon atoms, carried out in the presence of a catalyst component (A) comprising Mg, Ti and halogen as essential elements and of a catalyst component (B) capable to produce, under the same polymerization conditions, a polymer with an average particle size lower than that obtainable with the said catalyst component A. The said process provides polymers with increased bulk density.

[0001] The present invention relates to a process for the polymerizationof olefins CH₂═CHR, in which R is hydrogen or a hydrocarbyl radical with1-12 carbon atoms, directed to obtain polymer with increased bulkdensity, and to certain catalyst mixtures suited for the use in saidprocess. In particular the present invention relates to the use, in saidpolymerization process, of mixture of catalyst components able to formpolymer fractions with different average particle size. Ziegler-Nattacatalysts based on Mg, Ti and halogen are very well known in the art andare commonly used in the industrial plants for the olefinpolymerization. This ample category comprises specific grades ofcatalysts that, in dependence of their peculiarities are used in thepreparation of specific kinds of polymers. As a general rule, thedesired catalysts are those that are able to give the target polymerproperties while allowing the plant to keep a high productivity. One ofthe parameters hinting at a high productivity is the bulk density of thepolymer. Generally speaking, the higher is the bulk density of thepolymer the higher is the productivity of the plant. In certain caseshowever, it is not possible to use the catalysts capable to give highbulk density because either they are not able to impart to the polymersother desired properties or they are not suited to a given particularpolymerization process. This problem for example may arise when anheterophasic copolymer is to be produced, with the same catalyst, in twosequential polymerization step. An heterophasic polymer is a polymercomprised of a crystalline polyolefin phase (matrix) within which anamorphous phase (generally a propylene/ethylene and/or alpha-olefincopolymer) is dispersed. In this case, instead of a catalyst giving highbulk density, the suitable catalyst should have a certain porosity inorder to generate a porous crystalline matrix within which the amorphousphase can grow without giving rise to fouling phenomena. As a result,the target polymers are produced with a productivity of the plant belowthe maximum obtainable.

[0002] The applicant has now found that by employing specific catalystmixtures it is possible to enhance the bulk density of the polymers andtherefore the productivity of the polymerization processes while at thesame time retaining the desired properties of the polymers. It istherefore an object of the present invention a process for thepolymerization of olefins CH₂═CHR, in which R is hydrogen or ahydrocarbon radical with 1-12 carbon atoms, carried out in the presenceof a catalyst component (A) comprising Mg, Ti and halogen as essentialelements and of a catalyst component (B) capable to produce, under thesame polymerization conditions, a polymer with an average particle sizelower than that obtainable with the said catalyst component A.Preferably the average particle size of the polymer obtained with thecatalyst (B) is at least 25% lower than that of the polymer obtainedfrom (A) and more preferably at least 40% lower than that of the polymerobtained from (A). The catalyst component (B) can be chosen among thecatalyst components known in the art provided that it is capable to givethe polymer with the suitable diameter. Preferably, also the catalystcomponent (B) comprises Mg, Ti and halogen as essential elements. In afirst particular embodiment of the present invention the catalystcomponent (B) has substantially the same features as the catalystcomponent (A) except for a lower activity under the same polymerizationconditions. The said activity in particular is preferably at least 20%lower than that of (A) and more preferably at least 30% lower.

[0003] The applicant in fact, has found that the lower activity catalystduring the same polymerization time and conditions leads to a polymerwith a smaller particle size. Consequently, the overall bulk density ofthe polymer (as obtained from the catalyst components A+B) resultsincreased with respect of that obtainable by the use of (A) only. In asecond particular embodiment of the present invention, thepolymerization process is carried out with a catalyst mixture ofcatalyst components (A) and (B) both of them comprising Mg, Ti andhalogen as essential elements and characterized by the fact that (B) ispresent in an amount ranging from 1 to 60% b.w. of the total (A+B)preferably from 10 to 55% b.w. and has a lower average diameter withrespect to the catalyst A. Preferably, the difference between theaverage diameter of the two catalysts component is such that the averagediameter of the catalyst component fraction B is equal to, or lowerthan, 75% of the value of the average diameter of the catalyst componentfraction A. Preferably the average diameter of B is lower than −50% ofthe average diameter of A: In a preferred aspect of this embodiment theaverage particle diameter of the catalyst component B is from 5 to 60 μmand preferably from 5 to 40 μm while the range for the catalystcomponent A is from 30 to 200 μm and preferably from 30 to 120 μm andmore preferably from 30 to 90 μm. When the invention is operated underthis embodiment it is preferable that the two catalyst components havethe substantial same activity. The present invention is particularlyeffective when the catalyst components A and B have a narrow particlesize distribution (PSD). The breath of the PDS can be calculatedaccording to the formula $\frac{{P90} - {P10}}{P50},$

[0004] wherein P90 is the value of the diameter such that 90% of thetotal particles have a diameter lower than that value; P10 is the valueof the diameter such that 10% of the total particles have a diameterlower than that value and P50 is the value of the diameter such that 50%of the total particles have a diameter lower than that value. For thepurpose of the present invention, it would be preferable that both thecatalyst components A and B have a PSD calculated with the above formulalower than 1.8 and preferably lower than 1.2. In the case that thepolymerization process is directed, at least in part, to the preparationof a porous polymer, it is preferred using, in the second particularembodiment, a catalyst component (B) having a porosity, determined withthe mercury method, lower than that of the catalyst component (A) and inparticular within the range specified below. In a preferred embodimentof the present invention the catalysts A and B comprise titaniumcompounds having at least a Ti-halogen bond and a Mg dihalide. Themagnesium halide is preferably MgCl₂ in active form which is widelyknown from the patent literature as a support for Ziegler-Nattacatalysts. Patents U.S. Pat. No. 4,298,718 and U.S. Pat. No. 4,495,338were the first to describe the use of these compounds in Ziegler-Nattacatalysis. It is known from these patents that the magnesium dihalidesin active form used as support or co-support in components of catalystsfor the polymerization of olefins are characterized by X-ray spectra inwhich the most intense diffraction line that appears in the ASTM-cardreference of the spectrum of the non-active halide is diminished inintensity and broadened. In the X-ray spectra of preferred magnesiumdihalides in active form said most intense line is diminished inintensity and replaced by a halo whose maximum intensity is displacedtowards lower angles relative to that of the most intense line. Thepreferred titanium compounds used in the catalyst component of thepresent invention are the halides of Ti, in particular among those inwhich the Ti has valence 4, TiCl₄, and among those in which the Ti hasvalence lower than 4 TiCl₃; furthermore, can also be usedTi-haloalcoholates of formula Ti(OR^(I))_(n-y)X_(y), where n is thevalence of titanium, y is a number between 1 and n, X is halogen,preferably chlorine, and R^(I) is a C1-C15 hydrocarbon group optionallycontaining an heteroatom. In addition to the titanium compound and theMg dihalide, the catalysts components can also contain, and this isespecially preferred in the case of the preparation of stereoregularpolymers, one or more (internal) electron donor compounds. The electrondonor compound (d) can be selected from ethers, esters of organic monoor bicarboxylic acids, such as phthalates, benzoates, glutarates,succinates, ketones and amines. Preferably, it is selected from 1,3diethers of the type disclosed in EP 361494 and EP728769, and esters oforganic mono or bicarboxylic acids in particular aliphatic or aromaticphtahlates. Among this last class, particularly preferred compounds arethe alkyl esters of the phthalic acids.

[0005] The preparation of the solid catalyst components can be carriedout according to several methods known in the art. According to apreferred method, the solid catalyst component can be prepared byreacting a titanium compound of formula Ti(OR)_(n-y)X_(y), where n isthe valence of titanium and y is a number between 1 and n, preferablyTiCl₄, with a magnesium chloride deriving from an adduct of formulaMgCl₂.pROH, where p is a number between 0,1 and 6, preferably from 2 to3.5, and R is a hydrocarbon radical having 1-18 carbon atoms. The adductcan be suitably prepared in spherical form by mixing alcohol andmagnesium chloride in the presence of an inert hydrocarbon immisciblewith the adduct, operating under stirring conditions at the meltingtemperature of the adduct (100-130° C.). The average size of thedroplets of the molten adduct can be chosen for example by controllingthe stiring conditions. Generally, the more vigorous and intense is thestirring the lower is the average diameter of the droplets. When theemulsion is quickly quenched, the droplets of the adduct solidify inform of spherical particles having a size substantially corresponding tothat of the droplets. The control of the stirring and quenchingconditions also ensures that solid spherical adducts with having anarrow particle size distribution according to the present invention areobtained. Examples of spherical adducts prepared according to thisprocedure are described in U.S. Pat. No. 4,399,054 and U.S. Pat. No.4,469,648. The so obtained obtained adduct can be directly reacted withthe Ti compound or it can be previously subjected to thermal controlleddealcoholation (80-130° C.) so as to obtain an adduct in which thenumber of moles of alcohol is generally lower than 3 preferably between0,1 and 2,5. The reaction with the Ti compound can be carried out bysuspending the adduct (dealcoholated or as such) in cold TiCl₄(generally 0° C.); the mixture is heated up to 80-130° C. and kept atthis temperature for 0,5-2 hours. The treatment with TiCl₄ can becarried out one or more times. The internal electron donor can be addedduring the treatment with TiCl₄. The treatment with the electron donorcompound can be repeated one or more times. The preparation of catalystcomponents in spherical form is described for example in European PatentApplications EP-A-395083, EP-A-553805, EP-A-553806, EPA-601525 andWO98/44009. The solid catalyst components obtained according to theabove method show a surface area (by B.E.T. method) generally between 20and 500 m²/g and preferably between 50 and 400 m²/g, and a totalporosity (by B.E.T. method) higher than 0,2 cm³/g preferably between 0,2and 0,6 cm³/g. The porosity (Hg method) due to pores with radius up to10.000 Å generally ranges from 0.3 to 1.5 cm³/g, preferably from 0.45 to1 cm³/g.

[0006] The catalyst components with a very high surface area (over 300m²/g) can be obtained by directly treating the spherical solid particlesof the adduct with a dealcoholating agent such as TiCl₄. It is apreferred aspect of the present invention under the second particularembodiment that the solid catalyst component (B) having a lower averagediameter with respect to the catalyst component (A) is alsocharacterized by a higher surface area with respect to the catalystcomponent (A) and in particular by a surface area higher than 250 m²/g.The catalyst components having a high porosity determined with the Hgmethod can be obtained by reacting a titanium compound with a MgCl₂adduct disclosed above which has been subject to a thermal controlleddealcoholation treatment under hot gaseous stream. Higher porosity isgenerally obtained by removal of high amounts of alcohol from thestarting adduct. As previously mentioned, particularly when a porouspolymer is to be produced, it has been found suitable to have thecatalyst component (A) with a porosity (Hg method due to pores with adiameter up to 10,000 Å) higher than 0.6 cm³/g and preferably higherthan 1 cm³/g and, correspondingly, a catalyst component (B) with aporosity lower than (A) and in particular in the range 0.1-0.7. In anyof the preparation methods described above the desired electron donorcompound can be added as such or, in an alternative way, it can beobtained in situ by using an appropriate precursor capable to betransformed in the desired electron donor compound by means, forexample, of known chemical reactions such as esterification,transesterification etc. The solid catalyst components (A) and (B) areconverted into catalysts for the polymerization of olefins by reactingthem with suitable co-catalysts like the organometallic compounds of themetals belonging to groups 1-2 and 13 of the Table of Elements (newnotation) optionally in the presence of an external electron donor.Among organometallic compounds, organoaluminum compounds are preferred.

[0007] Particularly preferred are the alkyl-Al compound selected fromthe trialkyl aluminum compounds such as for example triethylaluminum,triisobutylaluminum, tri-n-butylaluminum, tri-n-hexylaluminum,tri-n-octylaluminum. It is also possible to use mixtures oftrialkylaluminum's with alkylaluminum halides, alkylaluminum hydrides oralkylaluminum sesquichlorides such as AlEt₂Cl and Al₂Et₃Cl₃. Theexternal electron donor can be of the same type or it can be differentfrom the internal electron donor compound present in the solid catalystcomponent. Suitable external electron-donor compounds include siliconcompounds, ethers, esters such as ethyl 4-ethoxybenzoate, amines,heterocyclic compounds and particularly 2,2,6,6-tetramethyl piperidine,and ketones. One particular class of preferred external donor compoundsis that of silicon compounds of formula R_(a) ⁵R_(b) ⁶Si(OR⁷)_(c), wherea and b are integer from 0 to 2, c is an integer from 1 to 3 and the sum(a+b+c) is 4; R⁵, R⁶, and R⁷, are alkyl, cycloalkyl or aryl radicalswith 1-18 carbon atoms optionally containing heteroatoms. Particularlypreferred are the silicon compounds in which a is 1, b is 1, c is 2, atleast one of R⁵ and R⁶ is selected from branched alkyl, cycloalkyl oraryl groups with 3-10 carbon atoms optionally containing heteroatoms andR⁷ is a C₁-C₁₀ alkyl group, in particular methyl. Examples of suchpreferred silicon compounds are methylcyclohexyldimethoxysilane,diphenyldimethoxysilane, methyl-t-butyldimethoxysilane,dicyclopentyldirnethoxysilane,2-ethylpiperidinyl-2-t-butyldimethoxysilane,1,1,1,trifluoropropyl-metil-dimethoxysilane and1,1,1,trifluoropropyl-2-ethylpiperidinyl-dimethoxysilane. Moreover, arealso preferred the silicon compounds in which a is 0, c is 3, R⁶ is abranched alkyl or cycloalkyl group, optionally containing heteroatoms,and R⁷ is methyl. Examples of such preferred silicon compounds arecyclohexyltrimethoxysilane, t-butyltrimethoxysilane andthexyltrimethoxysilane. The electron donor compound (c) is used in suchan amount to give a molar ratio between the organoaluminum compound andsaid electron donor compound (c) of from 0.1 to 500, preferably from 1to 300 and more preferably from 3 to 100. As previously indicated, thesaid catalyst are suitable for preparing a broad range of polyolefinproducts. They are particularly suitable for preparing linear lowdensity polyethylenes (LLDPE, having a density lower than 0.940 g/cm³)and very-low-density and ultra-low-density polyethylenes (VLDPE andULDPE, having a density lower than 0.920 g/cm³, to 0.880 g/cm³)consisting of copolymers of ethylene with one or more alpha-olefinshaving from 3 to 12 carbon atoms, having a mole content of units derivedfrom ethylene of higher than 80%. However, they can also be used toprepare, for example, high density ethylene polymers (HDPE, having adensity higher than 0.940 g/cm³), comprising ethylene homopolymers andcopolymers of ethylene with alpha-olefins having 3-12 carbon atoms;elastomeric copolymers of ethylene and propylene and elastomericterpolymers of ethylene and propylene with smaller proportions of adiene having a content by weight of units derived from ethylene ofbetween about 30 and 70%; isotactic polypropylenes and crystallinecopolymers of propylene and ethylene and/or other alpha-olefins having acontent of units derived from propylene of higher than 85% by weight;impact resistant polymers of propylene obtained by sequentialpolymerization of propylene and mixtures of propylene with ethylene,containing up to 30% by weight of ethylene; copolymers of propylene and1-butene having a number of units derived from 1-butene of between 10and 40% by weight. In view of the above, it constitutes a further objectof the present invention a process for the (co)polymerization of olefinsCH₂═CHR, in which R is hydrogen or a hydrocarbyl radical with 1-12carbon atoms, carried out in the presence of the catalyst describedabove. The olefins can be selected in particular from ethylene,propylene, butene-1,4-methyl-1-pentene, hexene-1, octene-1. Thepolymerization of propylene alone or in mixture with butene, hexene-1 oroctene-1 is especially preferred. The polymerization process in thepresence of catalysts obtained from the catalytic components of theinvention can be carried out according to known techniques either inliquid or gas phase using for example the known technique of thefluidized bed or under conditions wherein the polymer is mechanicallystirred.

[0008] Particularly preferred is the polymerization of propylene carriedout in liquid phase using, liquid propylene as polymerization medium.The catalyst of the present invention can be used as such in thepolymerization process by introducing it directly into the reactor.However, it constitutes a preferential embodiment the prepolymerizationof the catalyst with an olefin. In particular, it is especiallypreferred pre-polymerizing ethylene, or propylene or mixtures thereofwith one or more α-olefins, said mixtures containing up to 20% by moleof a-olefin, forming amounts of polymer from about 0.1 g per gram ofsolid component up to about 1000 g per gram of solid catalyst component.The pre-polymerization step can be carried out at temperatures from 0 to80° C. preferably from 5 to 50° C. in liquid or gas-phase. Thepre-polymerization step can be performed in-line as a part of acontinues polymerization process or separately in a batch process. Thebatch prepolymerization of the catalyst of the invention with ethylenein order to produce an amount of polymer ranging from 0.5 to 20 g pergram of catalyst component is particularly preferred. Examples ofgas-phase processes wherein it is possible to use the sphericalcomponents of the invention are described in WO92/21706, U.S. Pat. No.5,733,987 and WO93/03078. In this processes a pre-contacting step of thecatalyst components, a prepolymerization step and a gas phasepolymerization step in one or more reactors in a series of fluidized ormechanically stirred bed are comprised. Therefore, in the case that thepolymerization takes place in gas-phase, the process of the invention issuitably carried out according to the following steps:

[0009] (a) contact of the catalyst components in the absence ofpolymerizable olefin or optionally in the presence of said olefin inamounts not greater than 20 g per gram of the solid component (A);

[0010] (b) pre-polymerization of ethylene or mixtures thereof with oneor more a-olefins, said mixtures containing up to 20% by mole ofa-olefin, forming amounts of polymer from about 0.1 g per gram of solidcomponent (A) up to about 1000 g per gram;

[0011] (c) gas-phase polymerization of one or more olefins CH₂=CHR, inwhich R is hydrogen or a hydrocarbon radical having 1-10 carbon atoms,in one or more fluidized or mechanically stirred bed reactors using thepre-polymer-catalyst system coming from (b).

[0012] As mentioned above, the pre-polymerization step can be carriedout separately in batch. In this case, the pre-polymerized catalyst ispre-contacted according to step (a) with the aluminum alkyl and thendirectly sent to the gas-phase polymerization step (c). The molecularweight of the polymer is normally controlled using hydrogen or otheragents capable to regulate the Molecular Weight. If needed thepolymerization process of the invention can be performed in two or morereactors working under different conditions and optionally by recycling,at least partially, the polymer which is formed in the second reactor tothe first reactor. As an example the two or more reactors can work withdifferent concentrations of molecular weight regulator or at differentpolymerization temperatures or both. The following examples are given inorder to further describe the present invention in a non-limitingmanner.

[0013] Characterization

[0014] The properties are determined according to the following methods:

[0015] Melt Index: measured at 190° C. according to ASTM D-1238condition “E” (Goad of 2.16 Kg) and “F” (load of 21.6 Kg);

[0016] Porosity and surface area with nitrogen: are determined accordingto the B.E.T. method (apparatus used SORPTOMATIC 1900 by Carlo Erba).

[0017] Porosity and surface area with mercury for catalyst components:

[0018] The measure is carried out using a “Porosimeter 2000 series” byCarlo Erba

[0019] The porosity is determined by absorption of mercury underpressure. For this determination use is made of a calibrated dilatometer(diameter 3 mm) CD₃ (Carlo Erba) connected to a reservoir of mercury andto a high-vacuum pump (1·10⁻² mbar). A weighed amount of sample isplaced in the dilatometer. The apparatus is then placed under highvacuum (<0.1 mm Hg) and is maintained in these conditions for 20minutes. The dilatometer is then connected to the mercury reservoir andthe mercury is allowed to flow slowly into it until it reaches the levelmarked on the dilatometer at a height of 10 cm. The valve that connectsthe dilatometer to the vacuum pump is closed and then the mercurypressure is gradually increased with nitrogen up to 140 kg/cm². Underthe effect of the pressure, the mercury enters the pores and the levelgoes down according to the porosity of the material.

[0020] The porosity (cm³/g), both total and that due to pores up to10,000 Å, the pore distribution curve, and the average pore size aredirectly calculated from the integral pore distribution curve which isfunction of the volume reduction of the mercury and applied pressurevalues (all these data are provided and elaborated by the porosimeterassociated computer which is equipped with a “MILESTONE 200/2.04”program by C. Erba.

[0021] Porosity and surface area with mercury for polymers:

[0022] The same method and apparatus disclosed for the catalyst has beenused with the difference that the mercury pressure is graduallyincreased with nitrogen up to 2.5 Kg/cm².

[0023] Average Particle Size of the Catalyst

[0024] Determined by a method based on the principle of the opticaldiffraction of monochromatic laser light with the “Malvem Instr. 2600”apparatus. The average size is given as P50.

[0025] Average Particle Size of the Polymers

[0026] Determined through the use Tyler Testing Sieve Shaker RX-29 ModelB available from Combustion Engineering Endecott provided with a set ofsix sieves, according to ASTM E11-87, of number 5, 7, 10, 18, 35, and200 respectively.

EXAMPLES

[0027] Preparation of the Solid Catalyst Component A

[0028] Preparation of the Spherical Support (MgCl₂/EtOH Adduct)

[0029] The adduct of magnesium chloride and alcohol was preparedaccording to the method described in Example 2 of U.S. Pat. No.4,399,054, but operating at 900 rpm instead of 10,000 rpm. The adductcontains approximately 3 mol of alcohol. The alcohol was removed fromthe product thus obtained at temperatures that gradually increased from50° C. to 100° C. in nitrogen current until the alcohol content wasreduced to 2.1 moles per mole of MgCl2. The dealcoholated support had anaverage size of approximately 50 μM.

[0030] Preparation of the Solid Catalyst Component

[0031] Into a 2 L four-necked glass reactor, equipped with a mechanicalstirrer and a thermometer, purged with nitrogen, 1500 mL of TiCl₄ wereintroduced and cooled at 0° C. While stirring, 90 g of microspheroidalMgCl₂*2.1C₂H₅OH and diisobuthylphtalate was added, so that Mg/DIBP molarratio was 10.5. The temperature was raised to 100° C. and maintained for60 min. Then, the stirring was discontinued, the solid product wasallowed to settle at 100° C. for 15 minutes and the supernatant liquidwas siphoned off. Then 1500 mL of fresh TiCl₄ were added on the solidproduct. The mixture was reacted at 120° C. for 30 min and than thestirring was stopped and the reactor cooled to 100° C.; the solidproduct was allowed to settle at 100° C. for 15 min and the supernatantliquid was siphoned off. The solid was washed with 6×600 mL of anhydroushexane three times at 60° C. and three times at room temperature.Finally, the solid was dried under vacuum, analyzed and tested.

[0032] Preparation of the Solid Catalyst Component B

[0033] Preparation of the Spherical Support (MgCl₂/EtOH Adduct)

[0034] The adduct of magnesium chloride and alcohol was preparedaccording to the method described in Example 2 of U.S. Pat. No.4,399,054, but operating at 2500 rpm instead of 10,000 rpm. The adductcontaining approximately 3 mol of alcohol had an average size ofapproximately 21 μm.

[0035] Preparation of the Solid Catalyst Component

[0036] Into a 2 L four-necked glass reactor, equipped with a mechanicalstirrer and a thermometer, purged with nitrogen, 1500 mL of TiCl₄ wereintroduced and cooled at 0° C. While stirring, 75 g of microspheroidalMgCl₂*2.8C₂H₅OH and diisobuthylphtalate was added, so that Mg/DIBP molarratio was 13. The temperature was raised to 100° C. and maintained for60 min. Then, the stirring was discontinued, the solid product wasallowed to settle at 100° C. for 15 minutes and the supernatant liquidwas siphoned off. Then 1500 mL of fresh TiCl₄ were added on the solidproduct. The mixture was reacted at 120° C. for 30 min and than thestirring was stopped and the reactor cooled to 100° C.; the solidproduct was allowed to settle at 100° C. for 15 min and the supernatantliquid was siphoned off. The solid was washed with 6×600 mL of anhydroushexane three times at 60° C. and three times at room temperature.Finally, the solid was dried under vacuum, analyzed and tested.

Example 1

[0037] Preparation of the Solid Catalyst Component Mixture A/B andPolymerization Test

[0038] In a 4-liter autoclave, purged with nitrogen flow at 70° C. forone hour, 75 ml of anhydrous hexane containing 800 mg of AlEt₃, 56.4 mgof cyclohexylmethyldimethoxysilane, 7.9 mg of component A and 2.1 mg ofcomponent B were introduced in propylene flow at 30° C. The autoclavewas closed. 1.5 Nl of hydrogen were added and then, under stirring, 1.2Kg of liquid propylene were fed. The temperature was raised to 70° C. infive minutes and the polymerization was carried out at this temperaturefor two hours. The non-reacted propylene was removed, the polymer wasrecovered and dried at 70° C. under vacuum for three hours and, then,weighed. The BDP and the APS of the polymer were measured and reportedin Table 1.

Examples 2-4

[0039] The polymerization of Example 1 was repeated using the quantitiesof catalyst A and B reported in Table 1.

Comparison Example 1

[0040] The same procedure disclosed in Example 1 was repeated with thedifference that 10 mg of component A were used.

Comparison Example 2

[0041] The same procedure disclosed in Example 1 was repeated with thedifference that 10 mg of component B were used. TABLE 1 Amount ofPorosity Catalyst A/B B.D. A.P.S. (polymer) (polymer) Example (mg/mg)(g/cc) (μm) (cc/g) 1 7.9/2.1 0.466 1800 n.a 2 7.7/2.3 0.465 n.a n.a 35.9/4.1 0.481 1550 0.055 4 4.9/5.1 0.475 n.a n.a Comp. 1 10/0  0.4532100 0.09  Comp. 2  0/10 0.483  950 0.006

1. Process for the polymerization of olefins CH₂═CHR, in which R ishydrogen or a hydrocarbon radical with 1-12 carbon atoms, carried out inthe presence of a catalyst component (A) comprising Mg, Ti and halogenas essential elements and of a catalyst component (B) capable toproduce, under the same polymerization conditions, a polymer with anaverage particle size lower than that obtainable with the said catalystcomponent A.
 2. Process according to claim 1 in which the catalystcomponents (A) and (B) are reacted with organometallic compounds of themetals belonging to groups 1, 2 and 13 of the Table of Elements. 3.Process according to claim 1 in which the average particle size of thepolymer obtained with the catalyst (B) is at least 25% lower than thatof the polymer obtained from (A).
 4. Process according to claim 3 inwhich the average particle size of the polymer obtained with thecatalyst (B) is at least 40% lower than that of the polymer obtainedfrom (A).
 5. Process according to claim 1 in which also the catalystcomponent (13) comprises Mg, Ti and halogen as essential elements. 6.Process according to claim 1 in which the catalyst component (B) has alower activity with respect to (A).
 7. Process according to claim 6 inwhich the catalyst component (B) has an activity at least 20% lower thanthat of (A).
 8. Process according to claim 7 in which the catalystcomponent (B) has an activity at least 30% lower than that of (A). 9.Process according to claim 1 in which both of (A) and (B) comprise Mg,Ti and halogen as essential elements, (B) is present in an amountranging from 1 to 60% b.w. of the total (A+B) and is furthercharacterized by a lower average diameter with respect to A.
 10. Processaccording to claim 9 in which the average diameter of the catalystcomponent fraction B is equal to, or lower than, 75% of the value of theaverage diameter of the catalyst component fraction A.
 11. Processaccording to claim 9 in which the average diameter of B is equal to, orlower than, 50% of the value of the average diameter of A.
 12. Processaccording to claim 1 in which the catalyst components A and B have aPSD, according to the formula $\frac{{P90} - {P10}}{P50},$

wherein P90 is the value of the diameter such that 90% of the totalparticles have a diameter lower than that value; P10 is the value of thediameter such that 10% of the total particles have a diameter lower thanthat value and P50 is the value of the diameter such that 50% of thetotal particles have a diameter lower than that value, lower than 1.8.13. Process according to claim 1 in which the catalyst component (1) hasa porosity, determined with the mercury method, lower than that of thecatalyst component (A).
 14. Process according to claim 1 in which theolefin CH₂═CHR is selected from the group consisting of ethylene,propylene, butene-1, hexene-1, octene-1 and their mixtures.
 15. Catalystcomponent for the polymerization of olefins CH₂═CHR, in which R ishydrogen or a hydrocarbon radical with 1-12 carbon atoms, comprising atleast two catalyst component fractions A and B both of them comprisingMg, Ti and halogen as essential elements said catalyst componentcontaining from 1 to 60% b.w. of the fraction B which has a loweraverage diameter with respect to the catalyst A.
 16. Process accordingto claim 1 for the polymerization of propylene optionally in mixturewith butene, hexene-1 or octene-1, said process being carried out inliquid phase using, liquid propylene as polymerization medium. 17.Catalyst according to claim 16 characterized by the fact that thedifference between the average diameter of the catalysts componentfractions is such that the average diameter of the catalyst componentfraction B is equal to, or lower than, 75% the value of the averagediameter of the catalyst component fraction A.
 18. Catalyst componentaccording to claim 16 in which both the catalyst components A and B havea PSD calculated according to the above formula of claim 12 lower than1.8.
 19. Catalyst component according to claim 16 in which the averageparticle diameter of the catalyst component B is from 5 to 60 μm whilethe range for the catalyst component A is from 30 to 200 μm. 20.Catalyst component according to claim 16 in which both fractions (A) and(B) comprise a Ti halide or haloalcoholate of formulaTi(OR¹)_(n-y)X_(y), where n is the valence of titanium, y is a numberbetween 1 and n, X is halogen, and R¹ is a C1-C15 hydrocarbon groupoptionally containing an heteroatom, supported on a MgCl₂.
 21. Catalystcomponent according to claim 20 characterized by further comprising aninternal electron donor selected from the group consisting of ethers,esters of organic mono or bicarboxylic acids, ketones, amines and theirmixtures.
 22. Catalyst components according to claim 21 in which theinternal electron donor is an ester of organic mono or bicarboxylic acidselected from phthalates, benzoates, glutarates, and succinates. 23.Catalyst components according to claim 21 in which the internal electrondonor is a 1,3 diether.
 24. Catalyst components according to claim 16characterized by the fact that both e the catalyst fractions A and B areprepared by reacting a titanium compound of formula Ti(OR)_(n-y)X_(y),where n is the valence of titanium and y is a number between 1 and n,preferably TiCl₄, with an adduct of formula MgCl₂*pROH, where p is anumber between 0,1 and 6, preferably from 2 to 3.5, and R is ahydrocarbon radical having 1-18 carbon atoms.
 25. Catalyst componentsaccording to claim 16 characterized in that the solid catalyst componentfraction B has a porosity (Hg method due to pores with a diameter up to10,000 Å) lower than that of the catalyst fraction A.
 26. Catalystcomponents according to claim 25 in which catalyst component (A) has aporosity higher than 0.6 cm³/g and the catalyst component (B) has aporosity in the range 0.1-0.7.
 27. Catalyst for the polymerization ofolefins obtained by reacting a catalyst component according to claim 15with organometallic compounds of the metals belonging to groups 1-3 ofthe Table of Elements optionally in the presence of an electron donorcompound.